Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high-resolution image. An important part of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. Examples of color filter technology are shown in U.S. Pat. No. 6,297,071 and U.S. Pat. No. 6,274,917 (and the references cited therein). The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel.
Conventionally, micro-lenses are formed by spin coating a layer of micro-lens material onto a planarized layer. The micro-lens material is then etched to form cylindrical or other shaped regions that are centered above each pixel. Then, the micro-lens material is heated and reflowed to form a convex hemispherical micro-lens. FIG. 1 shows a prior art cross-sectional simplified diagram of an image sensor 101 having micro-lenses formed thereon. As seen in FIG. 1, the image sensor includes a plurality of pixels that have light detecting elements 103 formed in the substrate. The light detecting elements 103 may be one of several types, such as a photodiode, a photogate, or other solid state light sensitive element. Formed atop of each pixel is a micro-lens 105. The micro-lens 105 focuses incident light onto the light detecting elements 103. Moreover, in the region between the light detecting elements 103 and the micro-lens 105, denoted by reference numeral 107, there are various intervening layers that would typically include the color filter layers and various metal conducting lines.
The formation of the micro-lenses is a relatively precise process. In order to increase the efficiency of the light gathering aspect of the micro-lenses, the micro-lenses should be as large as possible. Still, it is undesirable for the individual micro-lenses to contact each other and thereby interfere. In sum, it is desired that the micro-lenses be as close to each other as possible without touching.
While this may be accomplished by precisely controlling the formation, size, and shape of the microlens cylindrical “blank”, as well as by controlling the reflow process, it is still difficult to ensure the formation of nearly perfect micro-lenses.